I think what Matthew is talking about is the "pop-up" illusion. When a visitor either clicks on the link or hovers over an area content suddenly becomes visible over top of the original content. This is done with a very long bit of Javascript. If you are feeling very good about your skills these guys have a pretty simple, easy to use example/sample code:
http://javascript.internet.com/navigation/absolute-popup-box.html .
Because it is such a complex code I prefer to use the pop-up control in the AJAX control toolkit with asp .net framework. It is much easier to implement and test.
If you're not a MS Visual Studio guy maybe one of the Linux geeks know if there is a php equivalent out there, although I'm doubtful (still waiting for the php version of masterpages.
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